An example of prior art image reading apparatus is disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 54-81715, for instance. FIG. 23 is a perspective view showing the prior art image reading apparatus disclosed in this document, and FIG. 24 is a cross-sectional view showing the carriage thereof, taken along the line B--B' in FIG. 23. In FIG. 24, an illumination unit 36 is composed of three image illuminating mercury fluorescent lamps of a red mercury fluorescent lamp 37, a green mercury fluorescent lamp 38, and a blue mercury fluorescent lamp 39, each having a predetermined distribution of spectral radiant energy and three voltage sources (not shown) for turning on respective lamps. These three red, green and blue mercury fluorescent lamps 37, 38 and 39 are turned on periodically and independently in sequence to illuminate an object 28 to be image read. The light reflected by the object 28 is image-formed on a line sensor 34 composed of a plurality of light-electricity transducing elements, through a reduction lens 33, in order to read the image by detecting chrominance signals, in sequence.
In the prior art fluorescent lamps, since the evaporation pressure of the mercury enclosed inside fluctuates according to ambient temperature, there exists a problem in that the quantity of light fluctuates according to the ambient temperature. In addition, in the case of the fluorescent lamp, the quantity of light obtainable is inevitably limited and further an appropriate tube diameter of the lamp is as large as 40 mm to maximize the luminous efficiency. Therefore, the volume of the three fluorescent lamps which constitute the color image scanner is large, and thereby it has been difficult to reduce the size of the illumination unit.
In addition to the above-mentioned problem, the prior art method involves another problem in the case where an object having sloped portions as with the case of an opened book is image read. In more detail, with reference to FIG. 25, when an object 28 having sloped portions is placed on a table 27 for image reading, the shade of colors which are not actually exist (referred to as shear in color image) is produced, thus resulting in a problem in that the color reproducibility is deteriorated markedly.
The causes of the above-mentioned problem are described in further details with reference to FIG. 25. When taking into account a point A at which the object 28 is not parallel to the table 27, the object at point A is illuminated sufficiently by the light emitted from the green and blue mercury fluorescent lamps 38 and 39, but not illuminated sufficiently by the light emitted from the red mercury fluorescent lamp 37. This is because the angle of incidence of light emitted from the red mercury fluorescent lamp 37 upon the object is small at point A. Therefore, in the case where the color of the object at point A is white, in spite of the fact that the quantity of light reflected from the object at point A must be equal to each other in the three colors of red, green and blue emitted by the respective mercury fluorescent lamps so that the line sensor 34 can generate the red, green and blue color output signals at the same level, since the red light emitted from the red mercury fluorescent lamp 37 is not sufficient at point A, the red color signal output of the line sensor 34 is low, with the result that the color at point A is reproduced in a purplish blue color.
In other words, the above-mentioned phenomenon occurs due to the fact that the spread angle a of mercury fluorescent lamp relative to an object, that is, the angle between the red mercury fluorescent lamp 37 and the blue mercury fluorescent lamp 39 obtained when seen from any given position on the object to be read is as large as 100 to 120 degrees in the case of the prior art illumination unit. The above-mentioned problem may be solved by collecting the three mercury fluorescent lamps at the same position to reduce the spread angle a to the object. In the case of the prior art illumination unit, however, since the optimum diameter of the fluorescent lamp is as large as 40 mm, it has been difficult to arrange the three fluorescent lamps at the same position.
Further, in the prior art illumination unit, since the three lamps are turned on periodically and independently in sequence, it is impossible to use the same single lamp activating power source, in common for the three lamps. As a result, when the prior art illumination unit is mounted on the carriage 35 of the image reading apparatus, a large space is inevitably required. In addition, since the carriage 35 is moved by the distance corresponding to the image-reading dimension of the object, there exists a problem in that the size of the image reading apparatus itself is inevitably increased to that extent.